A monopole antenna is a type of radio antenna formed by replacing one half of a dipole antenna with a ground plane at right-angles to the remaining half. If the ground plane is large enough, the monopole becomes similar to a dipole, as if its reflection in the ground plane forms the missing half of the dipole. Examples of monopole antennas include the whip antenna and the radio mast when isolated from the ground and bottom-fed.
The whip antenna is often a flexible springy wire mounted, usually vertically, with one end adjacent to a ground plane. A whip antenna can be a half element antenna that can be used with an unbalanced feed line such as coaxial cable, or attached directly to a wireless transmitter, receiver, or transceiver, in which case the radio case becomes the other dipole half element. The whip may actually form an asymmetric dipole rather than a ground-plane antenna. The short, flexible “rubber duck” antennas found on handheld two-way radios and cell phones may be examples of whip antennas, as are the long, flexible, stainless-steel antennas used in Citizens Band mobile installations. Some portable whip antennas can be telescoped down to a length of only few inches for transport and storage, and extended to several feet for operation.
Assets of the whip antenna include electrical and mechanical simplicity. Little or no installation is necessary. But, because many whip antennas are electrically small or operated with a poor electrical ground system, they can be inefficient.
In high-powered or long-range wireless communications, substantial outdoor antennas, used with well-engineered feed systems, work much better than whip antennas. In addition, such a transmitting antenna may be placed at a distance from humans and electronic equipment. A whip antenna structure may need to incorporate a transmission line.
The rigors of the military whip antenna environment cannot be understated. They should be capable of repeated and continuous flexing, as a result of soldier movement, without any damage to the antenna. In addition, they should be capable of withstanding the action of extensive exposure to sand, sea water and salt, ice, snow and the like.
The whip antenna approximates a 1 dimensional or line structure, taking up a minimum of volume. It is often the most operationally suitable antenna for lower frequencies, offering a good trade between radiation efficiency and physical size. Military manpack antennas are often used in combat areas where the requirement for low visibility is most important.
Since manpack battery power is small, antenna radiation efficiency must be high. The operator may stand, walk, or lie on the ground, which imposes demanding requirements on the antenna matching system. For instance, handset cord position varies antenna electrical structure, and an officer may even use a second handset. Proximity effects may occur as the soil and the radio operator both interact with the antenna near electric fields. Depending on tactical needs, manpack whip antennas may be bent or tucked into clothing, such as load carrying equipment (LCE/ALICE/MOLLE). Military radio men (“oscars”) have been creative in concealing the antenna whip, e.g. with some having even worn their radios upside down, inverting the antenna.
In United States military communications, the AT-272A “Blade” and AT-892/PRL-24 3-foot tape “Bush-Whip” are flexible whip antennas for manpack radios. Their mechanical construction may be familiar from measuring tapes, as they are made from tempered steel strips. Electrically they are monopole antennas but mechanically they are leaf springs. Leaf spring monopole antennas like the AT-272A and the AT-892/PRL-24 are rugged and durable in military service.
A limitation arises however when these whip or monopole antennas become electrically short: inductive loading is required and the associated loading “coil” can have resistive losses. In fact, if made short enough, all inductor loaded monopole antennas will become inefficient and have low gain. Larger and larger loading inductance is required with decreasing whip lengths, causing decreasing efficiency.
There is however a way to minimize loading losses in short monopoles: capacitive loading. This is because capacitors are generally more efficient than inductors. In current physics, nature provides excellent insulators but only fair conductors at room temperature. This situation favors capacitor efficiency over inductor efficiency. For instance, the “Q” value of available inductors may be only 100 at HF and but over 3000 for capacitors.
For capacitive loading to be used, the loaded antenna should provide an inductive driving point impedance, as the loading reactor must afford a conjugate match. Inductive driving impedances occur in small loop antennas, but loops can be physically unwieldy for portable radio communications. A whip antenna with inductive driving point impedance is needed.
U.S. Pat. No. 2,702,345, to Walter and entitled “Radiation and Interception of Electromagnetic Waves”, proposes inset or shunt feeds to accomplish linear antennas from various tower structures. There remains a need for military antennas mating to modern antenna couplers. Also, U.S. Pat. No. 3,403,405, to Barrar and entitled “Telescoping Folded Monopole With Capacitance At The Input” describes a mechanically reconfigurable antenna resonated with a single loading capacitor. Tuning is accomplished by adjusting antenna length, and a more durable construction could be desirable for military needs.
Prior art whip antennas include electrically short monopoles, operated by forced resonance, and mechanical tuning. They can include inductive loading with lossy loading coils, and radiation efficiency of less than 5%. A durable whip antenna for a radio, e.g. a manpack radio such as the Falcon Manpack Radio from Harris Corporation, is needed with a smaller size, higher gain and greater range.